doc: update reference to README.imximage

Update reference in many files detected by
scripts/documentation-file-ref-check

README.imximage => imx/mkimage/imximage.txt

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
diff --git a/board/freescale/mx6qarm2/imximage_mx6dl.cfg b/board/freescale/mx6qarm2/imximage_mx6dl.cfg
index 509c659..0d13531 100644
--- a/board/freescale/mx6qarm2/imximage_mx6dl.cfg
+++ b/board/freescale/mx6qarm2/imximage_mx6dl.cfg
@@ -3,7 +3,7 @@
  * Copyright (C) 2014 Freescale Semiconductor, Inc.
  * Jason Liu <r64343@freescale.com>
  *
- * Refer doc/README.imximage for more details about how-to configure
+ * Refer doc/imx/mkimage/imximage.txt for more details about how-to configure
  * and create imximage boot image
  *
  * The syntax is taken as close as possible with the kwbimage